Unless you want to be feeding more than 10lbs of feed a day, putting your horse on a senior feed isn't a good plan. Especially since your horse is competing and at 17 isn't likely to have the dental issues that would make a senior feed necessary. Look at senior feeds more after 20 yrs of age...
Instead, a good feed designed to meet the needs of performance horses would be a better choice. Strategy, Ultium....any number of things would be fine.
As far as the knees, I would have a vet examine them to determine what is causing the problem. The sooner you catch and treat the arthritic changes that are common in joints in competitive horses, the longer they are able to still function.
Licensed Veterinary Technician